Unless it's already registered as a SBR, a short barrel firearm with a pistol brace is not an SBR, it is a pistol. When you put a VFG on a pistol, it no longer meets the classification of a pistol, and becomes an AOW, which is illegal unless registered. BUT, if the OAL is greater than 26" and you put on a VFG, it no longer meets the classification of a pistol/rifle/AOW/SBR, making it only a firearm that is legal.

The measurement is from the muzzle of the barrel, or permanently affixed muzzle device, to the end of the stock/brace at it's fullest extension. I thought it was to the end of the buffer tube if you have a pistol brace, but lately this has become very confusing with adjustable length pistol braces, and could very well be to the end of the pistol brace at it's fullest extension.